1. Field of the Invention
The present invention relates to a synchronous control device for electric motors and machine axes driven by electric motors in a conveying device.
2. Description of the Related Art
When synchronizing a plurality of electric motors or machine axes driven by electric motors to keep the electrical phases thereof in the same relation to each other, a common rotational frequency reference is set, and correction of the synchronous control is made by using a deviation between the frequency signal corresponding to the rotational frequency reference and the frequency signal of a rotary encoder of the electric motor.
In the prior art, the signal formation and detecting method of the main control loop for the rotational frequency reference is independent from the correction loop for frequency deviation, so that a time lag between them occurs and highly accurate synchronous control is very difficult.
FIG. 7 shows a conventional synchronous control device for a plurality of electric motors, for example, two electric motors.
In FIG. 7, Cm is a concentrated control device of a master section. In the concentrated control device Cm, Sm is a rotational frequency setting device, Tm is an communication interface which sends out the rotational frequency setting signals from the rotational frequency setting device. The rotational frequency setting signals are sent to a slave section through a communication line 1.
Fm is a frequency signal generator which inputs the rotational frequency setting signals output from the rotational frequency setting device Sm and which generates frequency signals proportional to the rotational frequency setting input. The output from Fm is sent to the slave section through a signal line 2. Now, in the following explanation, a line which transmits signals transformed to the serial signals from the digital signals is called a communication line, and a line which sends pulse signals as they are is called a signal line.
Further, Cs1 and Cs2 are slave section control devices, As1 and As2 are driving devices of electric motors of the slave section, Ds1 and Ds2 are electric motors in the slave section, Rs1 and Rs2 are rotary encoders added to the electric motors, Gs1 and Gs2 are transmission devices, Ks1, Ks2 are machine axes driven by electric motors of the slave section.
The slave section control device Cs1 and Cs2 are composed of the same components, so that, in the following explanation, only the slave section control device Cs1 is explained, but the components of the slave section control device Cs2 are similarly numbered.
In FIG. 7, 11 is a communication interface, which receives the rotational frequency setting signals output from the communication interface Tm in the master section, and stores them into the rotational frequency reference storing means 12 as the master rotational frequency setting signals. 15 is a rotational frequency feedback detector, which detects feedback rotational frequency from the frequency signals output from the rotary encoder Rs1 in the slave.
13 is a frequency deviation counter, which up-counts the frequency signals from the frequency generator Fm in the master section, and detects the deviation by down-counting the frequency signals from the rotary encoder Rs1 in the slave section.
These outputs of the frequency deviation counter 13 are added or substituted to the master rotational frequency setting signals output from the rotational frequency reference storing means 12 through a proportional integration amplifier 14 (in the following, referred to PI) as correction signals, further calculated with the rotational frequency feedback signals output from the rotational frequency feedback detector 15 and sent to the driving device As1.
That is, the master rotational frequency setting signals are corrected based on the output of the frequency deviation counter 13 (frequency deviation), and rotational frequency and phase of the electric motor Ds1, Ds2 in the slave section Cs1, Cs2 are controlled based on the deviation between said corrected rotational frequency setting signals and rotational frequency feedback signals.
In the conventional method, the signal formation and the detecting method of the main control loop of the rotational frequency reference and the correction loop of the frequency deviation in the slave section are different, so that the time lag is inevitable and a highly accurate synchronous control is very difficult.
These are explained in FIG. 8 further.
In FIG. 8, 1 is a reference signal which is output from the communication interface Tm in the concentrated control device of the master section Cm in FIG. 7, received and detected in the communication interface 11 set in the rotational frequency slave section control device, and stored in the rotational frequency reference storing means 12. Further in FIG. 8, 2 is a frequency signal which is sent from the frequency generator Fm in the master section Cm in FIG. 7, and input to the frequency deviation counter 13 set in the slave section.
As shown in FIG. 8, the reference signal 1 and the frequency signal 2 are different in the signal form and the detecting method is also different, so that the time lag occurs although they should be overlapped in essence.
Moreover, in FIG. 8, times t1, t2, t3 . . . show timings to implement synchronous control processing in the slave section. At time t2, corresponding to a point A of the reference signal 1, the frequency signal 2 should agree with A'. However, the frequency signal 2 is controlled by using the value at B point, because of the time lag caused by the difference of a generated method and a transmission path between reference signal 1 and frequency signal 2. That is, .DELTA.F in FIG. 8 is occurred in the synchronous loop as an error in the synchronous control, so that a highly accurate synchronous control is difficult.